Method of firing steam generators with evaporated sulphite waste liquor



July 7, 1959 B. T. BRUN 5 METHOD OF FIRING STEAM GENERATORS WITHEVAPORATED SULPHITE WASTE LIQUOR Filed 001;. 29, 1957 WASTE SULF/TELIQUOR AND FLUX STEA/W AND FLUE GASES GENERATOR MOUL TEN ASH 8 2b FLUEGASES INVENTOR.

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United States Patent METHOD OF FIRING STEAM GENERATORS WITH EVAPORATEDSULPHITE WASTE LIQUOR Bengt Torsten Brunes, Djnrsholm, SwedenApplication October 29, 1957, Serial No. 693,193

1 Claim. (Cl. 110-1) The main object of the present invention is toprovide a method of firing steam generators by burning evaporatedsulphite waste liquor wholly or partially containing calcium as cation.In doing so, and if no special steps are taken against it, flue gaseswill be obtained which are intermingled with very large quantities ofextremely light ashes which involve considerable difliculties in thedesigning and operation of the steam generator. For instance, the lattermust be oversized considering the appreciable impairment in theabsorption of heat caused by ash deposits in the hearth and super heateretc. In order to reduce the risk of shutdowns, the steam generator mustalso be designed so as to obviate such deposits as far as possible and,in cases where they must inevitably arise they should be removed readilyand efiiciently, which substantially increases the cost of the plant. Inaddition, and

, almost without exception, an ash separator of a very efficient designmust be installed after the steam generator for separating the ashesmurder to avoid thereby sanitary inconveniences in the surroundings.

It has been suggested to provide a method of eliminating saidinconveniences by mixing the liquor with a flux agent before thecombustion, whereby the ashes of the liquor will precipitate or depositin a molten, liquid state and will be separated from the flue gasesbefore these are utilized in the steam generator.

A complete combustion then takes place while the total water content ofthe liquor will be evaporated simultaneously. For this reason thecontents of dry substance of the liquor must be comparatively high, atleast about 60 percent by weight if, when firing with liquor exclusivelyand with a reasonable addition of flux agent, such an elevatedcombustion temperature, up to 1450-1500 C., is to be achieved that theashes will melt into a state which is free-flowing enough for enablingseparation. However, in most cases, evaporation of the liquor to such ahigh degree of concentration is not economically or operativelysuitable, as far as the process of evaporation itself is concerned. Anoptimum usually accepted is about 50 percent, at which concentration thecombustion per se does not offer any unsolved problems. At thisconcentration, however, free-flowing ashes cannot be obtained, even ifthe liquor is unrestrictedly mixed with flux agent, at the combustiontemperature occurring, but the firing must be carried out with anadmixture of a more valuable auxiliary fuel, which in many cases ispractically unadvisable.

One object of the invention is to provide a method of performing thefiring in such a way that the ashes will be obtained from the combustionprocess in a molten, liquid state and separated from the flue gases evenon the assumption that liquor alone is burnt. Said liquor has not beenevaporated further than to the highest degree of concentration hithertoconsidered normal and/or with the addition of an amount of flux agentwhich is less than that hitherto required at a given concentration ofthe liquor supplied.

The invention is chiefly characterized in that the com- 2,893,333Patented July 7, 1959 bustion is performed in two consecutive stages ofcombustion, in each of which the liquor ashes formed being separatedfrom the flue gases before these are utilized in the steam generator,the liquor being burnt incompletely in the first stage at acomparatively low temperature, and the total or at least substantiallythe total water contents of the liquor being incorporated with the fluegases so that combustion residue will be formed which contains liquorashes and a considerable amount of unburnt liquor in a solid state. Thiscombustion residue is burnt in the second stage substantially withoutthe steam containing flue gases formed in the first stage being present,and the second stage combustion taking place at a considerably highertemperature than the first stage combustion. The liquor is mixed withsuch an amount of flux agent that the liquor ashes will assume a molten,liquid state during the combustionat said higher temperature.

An example of an arrangement for performing the invention is illustrateddiagrammatically in the accompanying drawing in a sectional elevation.

By means of outlets 2a and 2b, respectively, for flue gases, and bymeans of a furnace front 3, two turbulence or cyclone furnaces 1a and 1bare connected to the hearth 4 of a steam boiler, not shown in thedrawing. A number of tangential intakes 5 are provided for supplyingcombustion air into the respective furnaces. In addition, the furnace 1ais provided with an injection device 6 for liquor and a bottom outlet 7for combustion residues, and the furnace 1b is provided with anadditional tangential inlet 5a and a bottom outlet 8 for molten ashes.The furnace 1b is also provided with cooling coils 9 in the masonry. Aconduit 10 interconnects outlet 7 and inlet 5a.

In the furnace 1a the liquor is dried and ignited and partly burnt whilebeing caught by the air of combustion and driven spirally towards andalong the periphery of the furnace to the bottom 7 where it arrives inthe shape of a combustion residue of ashes and unburnt solid particlesseparated from the steam containing flue gases escaping through outlet2a. Through the conduit 10 this combustion residue is conducted to thefurnace 1b where it is completely burnt while turbulently passingthrough this furnace. In this course a small quantity of combustion gasfrom the furnace 1a may happen to flow along to the furnace 117 but thisquantity is so small that the combustion there in any case will takeplace virtually without such gas being present, so that the elevatedcombustion temperature may arise at which the ashes will melt into aliquid state in the presence of the flux agent added. The molten ashesmay be drained off through outlet 8, while separated from the flue gasesescaping through outlet 2b. In this case it is presupposed that theliquor is supplied to the furnace 1a mixed with a suflicient amount offlux agent, but it is also possible to add flux agent at a later pointbefore the final combustion, for instance in the conduit 10 or in thefurnace 1b.

The following numerical example of a process of combustion in the deviceillustrated may give some information about the possibilities of theinvention.

If a liquor having a dry substance proportion of about 47 percent byweight is incompletely burnt in the furnace 1a at a temperature. ofabout 800 C., a combustion temperature of 1650 C. may be attained at thesubsequent combustion in the furnace 1b, provided that about 10 percentof the heat supplied is lost by radiation, said combustion temperaturebeing higher than the temperature which may be expected in practice whenfiring a 60 percent liquor in one step in a furnace not cooled.

At this temperature, 1650 C., the ashes achieves a molten statesufliciently free-flowing for enabling the withdrawal if the liquor ismixed with flux agent in a quantity corresponding to about 2 percent NaO in the ashes at the most.

the other hand,ifthe 47 percent liquor isburnt in one step it is hardlypossible to reach a higher combustion temperature than 1250 C. .Anadmixture of flux agent correspondnig to as much as up to 6 or 8 percentNa O in the'ashes willpgive-themelting point 12 C.,whereas a-temperature250 C. higher, that is 1450 -C., has to be reached before the ashes willbe sufliciently -free flowing for being drained off, that is to say thateven if about four times as much flux agent is added as inthe combustionaccording to the present invention a one step combustion process willnot result in a sufliciently high temperature to enable the ashes to beseparated in a molten, liquid state.

As amatter-of course the arrangements for the combustion may be modifiedin many ways within the scope of the invention. -As regards the furnace1a it is likely that no better embodiment is available for the timebeing than the one illustrated. The furnace 1b on the other hand may bedesignedzaccording to many various .models available in the field ofmelt-combustion of dry fuels, such as coal. Neither is it necessary .toarrange the furnace one above the other in order that the combustionresidue .from the furnace 1a should be able to be transferred merely bygravityaction to the furnace 11;, but it is also conceivable that such atransfer may be accomplished by means of some positive conveyor device,such as a screw conveyor, injector or the like.

It is also possible, by maintaining a higher pressure in the furnace 1athan in furnace 1b, to cause a small part of the flue gas formed in thefurnace 1a to escape through the conduit .10 as a gas conveyor for thecombustion residue.

As anexample of the flux agent I may mention alkali salts, for instancesodium or potassium sulphate or carbonate, magnesium salts, such assulphate carbonate, sand etc. An addition of 2% by weight (calculated asNa,0 in the ashes) of said flux agent results in a melting point of1310' C. and an addition of 4% by weight (calculated as N830 in theashes) results in a melting point of 1250 C.

What I claim is:

A method of firing steam generators with a firing sys tem comprising twocombustion chambers by burning dry substance of sulphite waste liquor atleast partly containing calcium as cation and supplied with a flux agentto convert the ashes formed thereby into a molten liquid state,comprising the steps of burning incompletely in a first or saidcombustion chambers such liquor in a concentrated state therebyevaporating at least substantially the total water content of the liquorto produce a combustion residue in a solid powdered 'state, whichcontains ashes and a considerable amount of nonburned dry substance fromthe liquor, passing from said first combustion chamber the flue gasesand the vapor formed therein as heating medium to the steam generatorand passing the combustion residue to asecond of said combustionchambers, supplying additional combustion air'to said second combustionchamber so as to burn at least substantially completelythe nonburned drysubstance of said residue, and supplying the flux agent in an amountsufiicient to cause the residue of'ashes passed from the firstcombustion chamber and ashes newly formed in the second combustionchamber to assume a molten liquid state in that second combustionchamber at the combustion temperature-obtained therein, and passing fromthe second combustion chamber the flue gases formed therein as heatingmedium tothe steamgenerator andpassing the residue of molten ashes outof the firing system.

Colliau Feb. 1, 1887 Soderlund et a1. May 18, 1954

